In-service fuel cell performance recovery

ABSTRACT

The performance of a fuel cell power plant that decays, in an electric vehicle which makes frequent starts, is recovered by partially shutting down the power plant. Recovery is enabled by a recovery enable flag upon conditions such as vehicle using low or no power, vehicle speed at or near zero, electric storage SOC above a threshold, and no recovery during the last half-hour (or other duration). The recovery restart resets a timer to ensure that recovery is not attempted too often. The power plant then remains in a recovery stand-by mode until a recovery restart flag is set to 1. The restart causes start-up of the fuel cell power plant, reaching an operational mode.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/261,615, filed Mar. 12, 2013, which is a 371 of internationalApplication No. PCT/US2010/002501, filed Sep. 15, 2010, where eachapplication is incorporated by reference in its entirety.

TECHNICAL FIELD

Decayed performance of a fuel cell power plant operating a vehicle whichfrequently pauses, such as a bus, is restored (recovered) periodicallythroughout the time that the vehicle is in-service, the recoveryoccurring at times when the vehicle is stopped (idling).

BACKGROUND ART

Typical hybrid power plants for a city bus include a combination powersource that includes a battery pack and more recently a fuel cell stack.However, the fuel cell stacks which provide electric power for operatingbuses sustain daily decay in performance (voltage as a function ofcurrent density). Performance is recovered at the end of each day'sservice as part of a fuel cell power plant shutdown procedure. Theperformance of a fuel cell that provides motive power to a city bus isillustrated for the beginning of the day in the upper curve of FIG. 1and at the end of a nine-hour day in the lower curve of FIG. 1.

SUMMARY

It has been learned that the majority of daily performance decay in fuelcell stacks providing electric power to operate city buses occurs withinthe first hour of operation of the fuel cell stack. This means that suchfuel cell stacks are less efficient, and have lower voltage and lowerpower, most of the day.

In order to provide substantially maximum performance of a fuel cellstack powering a bus or other vehicle which makes frequent stops, aperformance recovery procedure is carried out periodically upon one ormore conditions, such as the average electric power produced in a recentinterval being less than a threshold, the vehicle speed being less thana threshold level, the battery state of charge exceeding a thresholdlevel, and the in-service performance recovery procedure not having beenprovided within a previous period of time, such as one-half of an hour.When performance recovery is enabled, the fuel cell power plant ispartially shut down, by removing the load and connecting the stack tovoltage limiting devices, stopping the flow of air to the cathodes, andstopping the flow of fuel. However, the temperature management system(coolant flow) remains in operation. The system will then hold, in aperformance recovery standby mode, until there is an indication that thefuel cell power plant should resume normal operation, e.g., the busbegins to move, demanding power, or the energy storage device needscharging.

In order to leave the performance recovery standby mode and restart theproduction of electricity by the fuel cell power plant, a recoveryrestart flag must be present. The routine which may generate this flagis performed, automatically, several times a minute. This flag isgenerated, for example, whenever the average power consumption of thevehicle drive system over a recent period, such as a minute, is greaterthan a threshold amount of power, or the vehicle speed exceeds a speedthreshold amount, or the state of charge of the battery (or otherelectric storage device) is low enough to require recharging.

The processes herein may be used with vehicles other than buses, such aspackage delivery trucks.

Other variations will become more apparent in the light of the followingdetailed description of exemplary embodiments, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a chart of fuel cell stack performance at the beginning of theday and at the end of a nine-hour day.

FIG. 2 is a functional diagram illustrating the selective generation ofa recovery enable flag.

FIG. 3 is a functional description of an alternative method ofgenerating a recovery enable flag.

FIG. 4 is a functional description of a method for generating a recoveryrestart flag.

FIG. 5 is a functional diagram of a fuel cell power plant control loop,illustrating a process of partially shutting down the fuel cell stack torecover performance of the stack.

MODE(S) OF IMPLEMENTATION

Conducting a performance recovery, and returning to normal fuel celloperation are both controlled by flags: a “recovery enable” flag and a“recovery restart flag”, respectively. How these flags relate to theoverall fuel cell control loop is described after the description of theflags themselves. Two alternative methods for generating the recoveryenable flag are illustrated in FIGS. 2 and 3; an embodiment forgenerating the recovery restart flag is illustrated in FIG. 4. Each ofthese flag programs are run periodically, such as three or four timesper minute. The recovery enable flag, once set to a 1, will causeperformance recovery to commence almost immediately. When performancerecovery has been accomplished, the recovery restart flag will beprovided only when the vehicle drive system demands more than athreshold amount of power, or the vehicle speed exceeds a low thresholdamount, or the battery needs recharging. Until one of these eventsoccurs, the fuel cell system waits in a recovery standby mode.

Referring to FIG. 2, a recovery enable routine is reached through anentry point 15 and a first test 16 determines whether the averageelectric power produced over a recent period, such as 30 seconds, isless than a recovery electric threshold, which may, by way of example,be about 5 kilowatts. If that is not the case, a negative result of test16 reaches a step 18 in which the recovery enable flag is set to zero.But if test 16 is affirmative, a test 19 is reached to determine if arecovery timer, which is reset at the end of each recovery procedure,has reached a time threshold, such as about 30 minutes, or such as othertime as is deemed suitable in any particular utilization, as a functionof the nature and condition of the fuel cell itself and the drive cycleexpected of the vehicle being powered. If test 19 is negative, therecovery enable flag is set to zero in step 18.

If test 19 is affirmative, a test 22 determines if the vehicle speed isless than a recovery low speed threshold, which may be on the order of0.1 miles per hour. If test 22 is negative, the step 18 will set therecovery enable flag to zero. If test 22 is affirmative, a test 23 willdetermine if the battery state of charge is greater than a recovery highstate of charge threshold, which may be on the order of 40%. If test 23is negative, step 18 will set the recovery enable flag to zero, but iftest 23 is affirmative, all of the tests 16-23 being affirmative, therecovery enable flag is set to 1 in a step 25. Then other programming isreverted to through a return point 27.

The embodiment of FIG. 2 requires that all four conditions be met beforerecovery enable can be set. However, in any given implementation, lessthan all four may be utilized, and other conditions may be utilized aswell.

FIG. 3 illustrates a variation of the embodiment of FIG. 2 which may beused as an alternative. In FIG. 2, if either test 16 or 22 isaffirmative, indicating either low power utilization or low speed, thenthe test 19 can be reached, and if affirmative, will set enable recoveryflag to 1 in step 25. But if both tests 16 and 22 are negative or iftest 19 is negative, then the step 18 is reached to set recovery enableflag to 0. Other variations may be made in the conditions forestablishing the recovery enable flag.

When performance recovery is accomplished, normal operation of the fuelcell power plant may be reestablished in response to a recovery restartflag, as is explained more fully with respect to FIG. 5, hereinafter. InFIG. 4, the routine for establishing the recovery restart flag isreached through an entry point 30, and a first test 32 determines if theaverage drive system power consumption over a recent time interval, suchas one minute, exceeds a recovery drive power threshold. If it does, anaffirmative result of test 32 reaches a step 35 to set the recoveryrestart flag to 1. If not, a test 34 determines if the vehicle speedexceeds a recovery low speed threshold. If so, the step 35 will set therecovery restart flag to 1. If not, a test 37 determines if the batteryneeds charging, by determining if the battery state of charge is lessthan a recovery low state of charge threshold. If so, the step 35 willset the recovery restart flag to 1. But if not, then all three tests 32,34 and 37 are negative which will reach a step 39 in which the recoveryrestart flag is set equal to 0. Then, another program is reverted tothrough a return point 40.

The conditions in FIG. 4 to determine whether the fuel cell power plantshould be restarted relate to the vehicle resuming motion, as indicatedeither by power consumption or speed in the tests 32 and 34 or simplythat the battery needs charging and therefore the fuel cell power plantshould be restarting.

An overall fuel cell power plant control loop utilizes the recoveryenable flag and the recovery restart flag to control performancerecovery of the fuel cell stack and to enable normal operation of thefuel cell power plant to be resumed when it is necessary either becausethe vehicle is moving or because of a need to charge the battery orother electric storage system.

In FIG. 5, there are three fuel cell power plant modes, includingstandby 43, which is when the fuel cell is turned off and the controllerof the fuel cell is waiting for a vehicle request for the fuel cellpower plant to run, as determined in a test 45. To get the fuel cellpower plant into operation, a first step may be to turn on thetemperature management system in a step 47. On the other hand, dependingupon other characteristics of the fuel cell power plant, not germane tothe disclosure herein, the temperature management system may not beturned on until later in the operation, as needed, or to supportreaching operating temperature, or for some other reason. A step 50causes the anode and recycle plumbing to be filled with fuel. A step 52causes the blower to be started filling the cathode with air. A step 55causes the main load switch to be closed.

Following the steps 47-55, the fuel cell power plant is in a normaloperation mode 57 where it is ready to supply any load from zero tomaximum load. The fuel cell power plant is then responsive to whateverdemand the vehicle makes for power. It will remain in the normaloperation mode 57 until either the vehicle request for the fuel cell torun is changed to 0, as indicated in a test 60, or until the recoveryenable flag is set to 1, as determined in a test 61. As long as bothtest 60 and 61 are negative, the fuel cell power plant remains in thenormal operation mode 57 to deliver power demanded by the vehicle. Ifeither the run request flag is 0 or the recovery enable flag is 1, thenone of the tests 60, 61 will be affirmative reaching a step 63 whichsets the recovery restart flag to 0 to ensure that the fuel cell powerplant may be shut down if the vehicle run request is 0 (test 60) and toensure that a restart will not occur until the recovery process iscomplete.

Recovery in this embodiment is achieved by the process of shutting downthe fuel cell power plant (except for the temperature managementsystem). This includes two steps 65 in response to which the main loadswitch is opened (disconnecting the load) and a voltage limiting deviceor devices are connected to the electrical output of the fuel cell powerplant. A step 66 turns off the blower, so that with no air and thevoltage limiting devices connected, the cathode potential is limited tobeing near hydrogen potential. Step 67 turns the fuel system off, sothat the fuel cell power plant is shut down.

A test 70 determines whether the control loop has reached this stage asa result of the recovery enable flag being equal to 1 or as a result ofthe vehicle run request flag being 0. If test 70 is negative, then theshutdown is due to the vehicle run request being 0. But if test 70 isaffirmative, the shutdown is as a result of a recovery being performed.An affirmative result of test 70 reaches a recovery standby mode 72where the fuel cell power plant control loop will remain until therecovery restart flag is 1, as indicated by a test 74. So long as test74 is negative, the fuel cell stack remains in the recovery stand-bymode 72. When the restart flag is set to 1, as described with respect toFIG. 4 hereinbefore, then the recovery enable flag is set to 0 in a step77 and the recovery timer is reset, so as to indicate in test 19 ofeither FIG. 2 or FIG. 3 when the present recovery was carried out.

When leaving the recovery standby mode 72 following steps 77 and 79, thefuel cell power plant control loop reverts to the three steps 50, 52, 55to establish operation of the fuel cell power plant and return it to thenormal operation mode 57 where the fuel cell is ready for any loaddemand between zero and maximum.

Should step 70 be negative, indicating that the vehicle run request flagwas set to 0, a step 80 will cause the temperature management system tobe turned off so that the fuel cell power plant is totally shut down.This causes it to be in the fuel cell standby mode 43 as describedhereinbefore, where it will remain until the vehicle requests it to runas indicated by test 45.

In this embodiment, the performance recovery is accomplished by thethree steps 65-67 which reduce cathode potential. However, otherembodiments not described herein fully may utilize other forms ofrecovery, including recycling cathode exhaust, providing hydrogen to thecathodes.

Since changes and variations of the disclosed embodiments may be madewithout departing from the concept's intent, it is not intended to limitthe disclosure other than as required by the appended claims.

1. A method of operating a fuel cell power plant that provides electricpower to an electrically propelled vehicle, the method comprising:recovering fuel cell stack performance when the electrically propelledvehicle is temporarily not moving and is demanding either a small amountof power or no power at all, wherein recovering the fuel cell stackperformance comprises, in order, disconnecting the electricallypropelled vehicle from an electric power output of the fuel cell powerplant, connecting one or more voltage limiting devices to the electricpower output of the fuel cell power plant, shutting off oxidant supplyto cathodes of the fuel cell power plant, and shutting off fuel supplyto anodes of the fuel cell power plant.
 2. The method according to claim1 further comprising: once fuel cell stack performance has beenrecovered, restoring normal operation of the fuel cell power plant inresponse to one or more conditions selected from a) a drive system ofthe electrically propelled vehicle consuming power in excess of athreshold amount, b) a vehicle speed exceeding a speed threshold, and c)an electric energy storage device which receives electric charge fromthe fuel cell power plant having a state of charge less than a thresholdstate of charge.
 3. The method of claim 1 wherein recovering fuel cellstack performance is triggered by a recovery enable flag, the recoveryenable flag being generated upon satisfaction of all of the followingcriteria: average electric power produced by the fuel cell power plantover a recent period is less than a recovery electric threshold; arecovery timer has reached a time threshold; a speed of the electricallypropelled vehicle is less than a recovery low speed threshold; and abattery state of charge is greater than a recovery high state of chargethreshold.
 4. The method of claim 3 wherein normal operation of the fuelcell power plant is reestablished in response to a recovery restartflag, the recovery restart flag being generated upon satisfaction of oneor more of the following criteria: an average drive system powerconsumption over a recent time interval exceeds a recovery drive powerthreshold; a speed of the electrically propelled vehicle exceeds arecovery low speed threshold; and a battery state of charge is less thana recovery low state of charge threshold.
 5. The method of claim 4,further comprising: upon generation of the recovery restart flag,resetting the recovery timer.
 6. The method of claim 1 whereinrecovering fuel cell stack performance is triggered by a recovery enableflag, the recovery enable flag being generated upon satisfaction of thefollowing criteria: average electric power produced by the fuel cellpower plant over a recent period is less than a recovery electricthreshold or a speed of the electrically propelled vehicle is less thana recovery low speed threshold; and a recovery timer has reached a timethreshold.
 7. The method of claim 6 wherein normal operation of the fuelcell power plant is reestablished in response to a recovery restartflag, the recovery restart flag being generated upon satisfaction of oneor more of the following criteria: an average drive system powerconsumption over a recent time interval exceeds a recovery drive powerthreshold; a speed of the electrically propelled vehicle exceeds arecovery low speed threshold; and a battery state of charge is less thana recovery low state of charge threshold.
 8. The method of claim 7,further comprising: upon generation of the recovery restart flag,resetting the recovery timer.
 9. The method of claim 1, furthercomprising: ensuring that normal operation of the fuel cell power plantwill not be reestablished until after the recovery of the fuel cellstack performance is complete.
 10. The method of claim 1, furthercomprising: after the recovery of the fuel cell stack performance iscomplete, determining whether the recovery of the fuel cell stackperformance was in response to a recovery enable flag or an absence of avehicle run request flag.
 11. A method comprising: when a vehicle,powered by electricity provided by a fuel cell power plant, istemporarily at rest and demanding little or no power from the fuel cellpower plant, recovering fuel cell stack performance, wherein recoveringfuel cell stack performance comprises, in order, disconnecting thevehicle from an electric power output of the fuel cell power plant,connecting one or more voltage limiting devices to the electric poweroutput of the fuel cell power plant, shutting off oxidant supply tocathodes of the fuel cell power plant, and shutting off fuel supply toanodes of the fuel cell power plant.
 12. The method according to claim11, further comprising: once fuel cell stack performance has beenrecovered, restoring normal operation of the fuel cell power plant inresponse to one or more conditions selected from a) a drive system ofthe vehicle consuming power in excess of a threshold amount, b) avehicle speed exceeding a speed threshold, and c) an electric energystorage device which receives electric charge from the fuel cell powerplant having a state of charge less than a threshold state of charge.13. The method of claim 11 wherein recovering fuel cell stackperformance is triggered by a recovery enable flag, the recovery enableflag being generated upon satisfaction of all of the following criteria:average electric power produced by the fuel cell power plant over arecent period is less than a recovery electric threshold; a recoverytimer has reached a time threshold; a speed of the vehicle is less thana recovery low speed threshold; and a battery state of charge is greaterthan a recovery high state of charge threshold.
 14. The method of claim13 wherein normal operation of the fuel cell power plant isreestablished in response to a recovery restart flag, the recoveryrestart flag being generated upon satisfaction of one or more of thefollowing criteria: an average drive system power consumption over arecent time interval exceeds a recovery drive power threshold; a speedof the vehicle exceeds a recovery low speed threshold; and a batterystate of charge is less than a recovery low state of charge threshold.15. The method of claim 14, further comprising: upon generation of therecovery restart flag, resetting the recovery timer.
 16. The method ofclaim 11 wherein recovering fuel cell stack performance is triggered bya recovery enable flag, the recovery enable flag being generated uponsatisfaction of the following criteria: average electric power producedby the fuel cell power plant over a recent period is less than arecovery electric threshold or a speed of the vehicle is less than arecovery low speed threshold; and a recovery timer has reached a timethreshold.
 17. The method of claim 16 wherein normal operation of thefuel cell power plant is reestablished in response to a recovery restartflag, the recovery restart flag being generated upon satisfaction of oneor more of the following criteria: an average drive system powerconsumption over a recent time interval exceeds a recovery drive powerthreshold; a speed of the vehicle exceeds a recovery low speedthreshold; and a battery state of charge is less than a recovery lowstate of charge threshold.
 18. The method of claim 17, furthercomprising: upon generation of the recovery restart flag, resetting therecovery timer.
 19. The method of claim 11, further comprising: ensuringthat normal operation of the fuel cell power plant will not bereestablished until after the recovery of the fuel cell stackperformance is complete.
 20. The method of claim 11, further comprising:after the recovery of the fuel cell stack performance is complete,determining whether the recovery of the fuel cell stack performance wasin response to a recovery enable flag or an absence of a vehicle runrequest flag.